The present invention relates to an amplifier circuit and more particularly to an amplifier circuit for accepting a low level, high impedance output signal from a photodiode and then amplifying this signal to a voltage and power level suitable for interfacing with standard circuitry while introducing a minimum of signal distortion.
One problem in transmitting information through standard electrical cables is the undesirable effect of radiated and conducted EMI (electro-magnetic interference) normally associated with these cables. Such EMI may, for example, cause spurious or other erroneous readings from equipment attached to the cable. One solution to this problem has been to convert the information to be transmitted from electrical energy to light energy, transmit the light energy signal through a light conducting cable such as, for example, a fiber optic bundle, and reconvert the light energy into electrical energy at the receiving end. Since the light conducting cable is not responsive to EMI, interference on the line is thus eliminated.
Fiber optic technology is being considered for future applications in two distinct areas of the data communication field. The first of these application areas, which is presently under intensive development, is concerned with the transmission of data over medium to long distances. Long distance data transmission utilizes low loss optical fiber, avalanche photodiode detectors with special low noise preamplifiers and bias stabilization circuits, thermoelectrically cooled laser diodes, and periodic repeater stages. Because of the potentially large commercial application for long distance optical data transmission, much privately sponsored research is directed towards this area. Medium and long distance optical communication links are also of considerable interest to the military departments.
The second area of development is concerned with the optical transmission of data over short distances of a few hundred meters or less. The advantages to be gained over the use of conventional wire cables include: high per channel data rate capability, immunity to electro-magnetic interference, lower cable weight, elimination of fire hazard due to electrical shorting, and potentially lower cost. For short length data link applications, multi-fiber bundles of medium and high loss fiber are utilized. Light emitting diodes (LED's) are employed as optical sources, and photodiodes are used for optical detection. Short distance optical data transmission is of particular interest to the military departments since this technology has been proposed for the optical wiring of aircraft where line lengths of 150 feet or less are encountered.
The technology required for the application of fiber optic data transmission systems to military equipment is in the early feasibility stage of development. At present, considerable effort is being expended towards defining and developing the components and systems needed for the implementation of fiber optics data links. Much progress is still required in all spects of fiber optics technology before reliable, large scale applications can be made to military systems.
The detection of "light" (visible or infrared radiation) signals in a fiber optics data transmission system is accomplished by use of a silicon photodiode. The photodiode is heavily reverse biased, and its equivalent circuit can be represented as a light dependent current source, shunted by the depletion layer capacitance. This simplified equivalent circuit would also include a small series resistance. In most fiber optic receiver applications, the diode signal current is fed directly into a transimpedance type amplifier. The diode output is terminated at the virtual ground created by the amplifier. If an ideal amplifier is assumed, the virtual ground point appears as a perfect short circuit to the diode. Under this hypothetical condition the frequency response of the receiver is determined by the photodiode short circuit response, and by the time constant of the amplifier feedback network. In the case of a real receiver, the less than ideal gain-frequency characteristics of the amplifier introduce some additional frequency response limitations.
In most applications, a feedback circuit is provided with the amplifier and a feedback resistance is provided which is made relatively large in order to achieve significant amplification. With this relatively large value of feedback resistance, only a few picofarads of shunt capacitance results in a sizable time constant.
The principal limitation of the common commercial circuit implementation is therefore due to conflicting requirements to make the feedback resistance large for reasons of gain, and at the same time as small as possible in order to meet bandwidth requirements. These constraints are imposed on the circuitry by the basic characteristics of the photodiode which are high output impedance and a low output signal current level. A tradeoff decision is required in the amplifier design between gain and bandwidth. An alternate approach to achieving large bandwidth response involves a lowering of the feedback resistance, and the utilization of a cascaded string of wide bandwidth amplifiers. This latter design approach is, however, complicated, expensive, and sensitive to environmental changes.